Earthworms: Oligochaeta

views updated

EARTHWORMS: Oligochaeta

RIVER WORM (Diplocardia riparia): SPECIES ACCOUNTS
GIPPSLAND GIANT WORM (Megascolides australis): SPECIES ACCOUNTS

PHYSICAL CHARACTERISTICS

The segmented bodies of earthworms measure up to 19.68 feet (6 meters) and resemble a tube within a tube. The outer body wall is made up of two muscle layers. The outer layer is made up of a series of circles wrapped around the body, while the inner layer of muscle runs along the length of the body. This inner layer shortens or extends the length of the body. Inside the body is the digestive tract, a tube that runs from the mouth, where food is taken in, to the anus (AY-nuhs), where waste, undigested food, and other particles leave the body. Between the muscular body wall and the digestive tract is the body cavity, where all the other organs are located. These organs are usually organized into body segments, just like the outer body. Earthworms do not have flaplike structures to help them move. All but the first body segment is covered with small, stiff bristles, or chaetae (KEY-tee), that help earthworms to hold position as they burrow through the soil. Toward the front of the body is a swollen, collarlike band called the clitellum (KLAI-teh-lum). Special tissues in the clitellum produce a collarlike egg case called a cocoon. These tissues also produce food for the eggs as they develop.

GEOGRAPHIC RANGE

Earthworms are found worldwide. They do not occur in deserts, polar regions, or in strongly acid soils. A few species have been widely distributed by humans.

HABITAT

Most earthworms live in the soil, but some prefer the mud along the shores of fresh or salty bodies of water. Depending on species, many earthworms live in the upper leaf litter layer, topsoil, or in deeper layers in the soil. Others live high above the forest floor in soils that accumulate among the branches of tree canopies in tropical rainforests.

DIET

Earthworms eat dead and decomposing leaves, decaying roots, and other bits of plant material, or detritus (dih-TRY-tuhs), in the soil. Worms living in leaf litter layer, upper soil, or in soils in tree canopies eat freshly dead plant materials. Other species, such as the European night crawler, always live in deep burrows. They come up to the surface to also eat freshly dead plant tissues. Still other species remain deep in the soil, where they eat long-buried bits of detritus.

BEHAVIOR AND REPRODUCTION

Earthworms defend themselves in a variety of ways. Those living in tunnels escape danger by quickly withdrawing into their burrows. Those living near or on the surface have the ability to move quickly. They jump and thrash wildly about and will even purposely break off a few tail segments when threatened by a predator. Species living deep in the soil have few defensive behaviors. Some will twist and coil their bodies or produce bad smelling or tasting fluids.

Earthworms have both male and female reproductive organs. Individuals in cooler climates mate in spring or fall to exchange sperm. Many tropical species are active during the rainy season. Worms align themselves in opposite directions and place sperm directly into the other's body. The collarlike cocoon produced by the clitellum passes over the female reproductive openings to receive one or more eggs, and then over the male openings to receive sperm. Fertilization takes place inside the cocoon.

Cocoons are deposited in the soil soon after mating. The developing eggs are nourished inside the case with materials also produced by the clitellum. The young earthworm passes through several stages before emerging from the cocoon as a small earthworm. Depending on the species, young earthworms may take several months or years to reach adulthood.

EARTHWORMS AND PEOPLE

Earthworms are very important because they improve and maintain soils, helping plants to grow. Their burrowing activities not only allow air into the soil, but also provide food for growing plants by mixing soil with bits of plant materials. Several species of earthworms are raised to collect their waste for use as a high-quality compost for gardening. Others species are raised and sold as fish bait.

CONSERVATION STATUS

Seven species of earthworms are listed by the World Conservation Union (IUCN). The Australian Lake Pedder earthworm is listed as Extinct, or no longer living. Phallodrilus macmasterae from Bermuda is listed as Critically Endangered, or facing an extremely high risk of extinction in the wild. Three of the four species considered Vulnerable live in the United States. Vulnerable means the species are facing a high risk of extinction in the wild. These include the American giant Palouse earthworm from Oregon, the Washington giant earthworm from Washington state, and Komarekiona eatoni that lives in the Midwest. The fourth species is the Gippsland giant worm of Australia. It is the only species that is clearly protected.

MR. DARWIN'S CAN OF WORMS

The famous scientist Charles Darwin (1812-1882) spent 40 years observing and studying the behavior of earthworms, mostly in his garden. He was the first to see their role in making soil more fertile. He estimated that in a single acre every year the worms could haul up 7 to 18 tons of soil to the surface as body waste, or castings. Darwin's last book, The Formation of Vegetable Mould through the Action of Worms with Observations on Their Habits, was published in 1881, just six months before his death.

Another American species from Louisiana, Lutrodrilus multivesiculatus, is listed as Lower Risk, or at risk of becoming threatened with extinction in the future. These and other earthworms are threatened by loss of habitat and by the fact that many live only in a few places. For example, the grassland habitats of many species have been converted into farmland.

RIVER WORM (Diplocardia riparia): SPECIES ACCOUNTS

Physical characteristics: The river worm has a dark brown body 4.7 to 7.8 inches (120 to 200 millimeters) in length. It has small pairs of lengthwise grooves underneath the body.

Geographic range: The river worm is found in the Central United States, including Iowa, Illinois, Missouri, Kansas, and Nebraska.

Habitat: River worms live in fine soils washed up along river banks and beneath stands of silver maples.

Diet: They burrow through leaf litter and just below the soil surface, eating detritus buried in river mud.

Behavior and reproduction: Nothing has been written about their behavior. River worms have both male and female reproductive organs.

River worms and people: River worms are collected and sold for fish bait. This species is more tolerant of summer heat than other bait species.

Conservation status: This species is not considered endangered or threatened. However, exotic species brought in as fish bait have become established along some rivers and streams, crowding out the river worm. ∎

GIPPSLAND GIANT WORM (Megascolides australis): SPECIES ACCOUNTS

Physical characteristics: The head of the Gippsland giant worm is dark purple. The rest of the body is pinkish gray. They measure up to 31.5 to 39.3 inches (80 to 100 centimeters) in length and have small pairs of markings underneath the body.

Geographic range: The Gippsland giant worm lives only in the Bass River Valley of Victoria, Australia.

Habitat: This species burrows in clay soils along streams and in other moist, but not too wet, habitats.

Diet: This species eats detritus in soil.

Behavior and reproduction: The giant worm's entire life, including feeding, mating, and waste deposition, occurs underground. They burrow down to the layer of soil that is soaked with water.

Cocoons are deposited underground, and eggs develop for 12 to 14 months. The larvae (LAR-vee), or animals in an early stage that change form before becoming adults, emerge from the cocoons measuring 7.8 inches (20 centimeters) in length.

Gippsland giant worms and people: The Gippsland giant worm is one of the largest earthworms in the world and a wonder of nature. Tourists flock to the Giant Worm Museum in Bass and to the worm festival in the town of Koramburra.

Conservation status: This species is listed as Vulnerable by the World Conservation Union (IUCN). This means that the Gippsland giant worm faces a high risk of extinction in the wild. It lives only in a very small area and is threatened by land development for farming and reduced water levels in the soil. ∎